With the recent promotion of conservation of energy, a toner which is fixed at a low temperature as a property required for the toner is demanded. However, if a binder resin having a low glass transition temperature for a toner is selected for achieving low-temperature fixability, the storage stability of the toner is inevitably deteriorated. In order to solve this problem, a method in which a toner is encapsulated by coating a toner surface with a resin having a high glass transition temperature or a resin having a crosslinked structure is being studied.
As representative examples of the toner encapsulation method, there are: a method of attaching and fusing resin particles to surfaces of toner particles; and a method of reacting a polymerizable monomer on surfaces of toner particles.
As a conventional method of attaching and fusing resin particles to surfaces of toner particles, there is known a following technique. Toner component particles are aggregated to form core particles in water, and then, fine particles for a shell are attached thereto to effect coating, followed by melting the fine particles through heating, whereby a toner is obtained. According to this method, there is a possibility that both low-temperature fixability and storage stability can be achieved by preparing the shell particles having a higher thermal characteristic than the core particles, but since the size of the shell particles is about 0.1 μm, a formed shell layer becomes relatively thick, and therefore, the resultant toner is liable to have an inferior low-temperature fixability.
On the other hand, it is known to react a polymerizable monomer on surfaces of toner particles. In this method, the particles are coated with a urea resin by an in situ polymerization method, and an extremely thin shell layer can be formed. However, since a monomer having a low molecular weight is used, the resulting resin is caused to have a dense crosslinked structure, and therefore, the resultant toner is liable to have an inferior low-temperature fixability. Further, the toner tends to have a poor chargeability, and moreover, the toner is accompanied with a problem due to residues of formaldehyde used as the monomer.